Compressor



10, 1939. c CAMPBELL 2 14:1180

COMPRESSOR Filed Oct. 27, 1937 57w JE 4501 QTZTZQ 3 4% 93:

Snventor (Ittornegs @ewewowewn baea Patented Jan. 10, 1939 UNITED STATES PATENT OFFICE COMPRESSOR Application October 27, 1937, Serial No. 171,335

6 Claims.

This invention relates to air compressors and particularly to cross compound steam driven two stage compressors of the type extensively used on steam locomotives to charge the main reservoir of the air brake system.

The principal object of the invention is to facilitate starting of the compressor, and particularly to prevent stalling of the compressor, when the governor acts to start it after a shut down.

In such compressors the steam end is doubleacting and cross compound, the valve gear for both steam cylinders being tripped by the high pressure piston as it approaches its two limits of stroke. The air end is two stage and comprises two double acting cylinders with first stage inlet valve, interstage valves, and high pressure discharge valves, all of the automatic (poppet) type and all set to close by gravity. The high pressure steam piston drives the large, first stage (low pressure) air piston, and moves reversely with respect to the low pressure steam piston which drives the second stage (high pressure) air piston. The pistons of the two stages are not mechanically connected, but their motion is timed by the steam valve gear.

Such compressors have long been in extensive use. Their outstanding merit has been the utter reliability, so necessary in railway brake service. Recently improved workmanship, such as lapped cylinders and special tight sealing piston rings, have markedly improved performance. Unfortunately introduction of these improvements was soon followed by numerous complaints as to compressors stalling. Examination of such stalling compressors showed them to be in perfect working order. Since such stalling is often seriously dangerous and at best causes intolerable trafiic delays, immediate solution of the difficulty was necessary.

The present invention is based on the discovery by another inventor of the cause of such stalling and is directed to certain features which permit quick and inexpensive conversion of existing compressors, and also improve the general operative characteristics.

If, when the compressor is stopped, a high pressure air discharge valve fails to seat tightly for any reason, and they occasionally do so, the main reservoir air (at say 140 pounds gage) feeds back to the high pressure cylinder and loads the intermediate valves so that they cannot open until pressure in the low pressure cylinder attains an equal value. If the compressor has stopped near the commencement of the compression stroke in one of the low pressure air working spaces the high pressure steam piston will move the low pressure air piston only part stroke, for the development of '70 pounds gage in the low pressure air cylinder will balance the boiler pressure commonly used in the high pressure steam cylinder. Hence the high pressure steam piston cannot move far enough to trip the valve gear, with the result that the compressor stalls.

With the somewhat leaky air pistons formerly used, air escaped around the piston and the compressor started after slight delay. With modern tight pistons a stalled compressor would not start at all. The usual, but very objectionable expedient, was to bleed the main reservoir, whereupon the compressor would start and would continue to operate satisfactorily for an unpredictable period, 1. e., until a discharge valve failed to seat tightly as the pump stopped.

v To correct this defect it has been proposed to use loaded relief valves for venting the working spaces of the low pressure air cylinders if these become overloaded. With a normal low pressure discharge pressure of the order of 40 pounds gage, and a stalling pressure of the order of '70 pounds gage, the relief valves can be set to open at some chosen value between these pressures, say 50 pounds gage. These valves remain closed when the compressor operates normally but open temporarily during astart if the intermediate valves are loaded. They have been found in actual service to eliminate stalling without loss of efficiency.

Since the relief valves are not intended for dissipating dangerous pressures, they may be and preferably are made of small flow capacity, to the end that failure of a relief valve will not disable the pump completely. This detail is also the invention of another.

In connection with the restricted flow principle, special construction is used to prevent squealing and ensure quiet operation.

Other features of invention reside in specific details of construction, such as mounting the relief valve in the inlet valve cap for one end of the cylinder, and in an intermediate valve cage for the other end of the cylinder, such arrangements offering simple modes of applying the relief valves without change in the design of existing compressors.

The invention will now be described in detail in connection with the accompanying drawing, in which:

Fig. 1 is a diagrammatic section of a cross compound two stage air compressor showing the invention applied.

Fig. 2 is a axial section through an intermediate valve cage with relief valve applied.

Fig. 3 is an axial section of a low pressure inlet valve cap with the invention applied.

Referring first to Fig. 1, 6 represents the high pressure steam cylinder, and the low pressure steam cylinder of a cross-compound unit for driving a compressor. 8 is the double-acting high pressure steam piston and 9 is the doubleacting low pressure steam piston. The high pressure piston 8 is connected by rod II with the low pressure or first stage air piston I2 which works in the low pressure air cylinder l3. The low pressure steam piston 9 is connected by the rod M with the high pressure (second stage) air piston |5 which works in the high pressure air cylinder l6. H is the steam supply connection which leads to the main steam distributing valve I8. This valve is shifted between its two limiting positions by steam pressure under the control of a pilot valve IS. The pilot valve I9 is shifted between its two limiting positions by a rod 2| having a shoulder 20, and terminating at its lower end in a button 22. A plate 23 attached to the high pressure steam piston 8 embraces rod 2|, and coacts with button 22 and shoulder 20 to shift rod 2| near the opposite ends of the piston stroke.

The steam valve mechanism is shown in position to cause the downward excursion of the high pressure steam piston 8 and the simultaneous upward excursion of the low pressure steam piston 9. The high pressure piston 8 is shown in the position in which the plate 23 is just about to pick up the button 22 and shift the pilot valve |9 downward. Downward shifting of the pilot valve l9 causes the shifting of the main distributing valve |8 to its extreme right hand position. Such reversal causes the upward excursion of the high pressure steam piston and the simultaneous downward excursion of the low pressure steam piston. The porting is clearly shown in diagram, and there is nothing novel in the steam end of the pump illustrated. For this reason, and for the further reason that pumps of this type are very well known in the art, it seems unnecessary to describe in detail the porting of the steam end, beyond pointing out that the exhaust passage is indicated at 25.

In pumps of this type the steam valve mechanism is in a steam head 26 common to the two steam cylinders 5 and i. A so-called center plate 21 serves as spacing means between and as heads for the lower ends of the steam cylinders 5 and l, and upper ends of the air cylinders I3 and Hi. This center plate carries packing glands 2B for the piston rods H and I4, and also the low pressure inlet valve, and the intermediate valves (two, arranged in parallel) for the upper working space of the air compressor. A lower head 29 closes the lower ends of the air cylinders and carries the inlet valve and the intermediate valves (two, arranged in parallel) for the lower working space of the air compressor.

Since all air valves are of the automatic poppet type, and are arranged to seat by gravity, the porting for the valves of the upper and lower working spaces of the compressor differs in form (though not in function). This entails structural differences in the application of the relief valves for the two ends of the low pressure air cylinder.

Generally stated, for the upper working space, the low pressure inlet valve and the intermehousing 3i.

diate valves are mounted under removable screw caps, and the cap, over the inlet valve is readily availed of to carry the relief valve. For the lower working space the corresponding valves are mounted in removable cages. In consequence, the relief valve cannot conveniently be mounted in the inlet valve cage, but can very simply be mounted on the cage of one of the intermediate valves. The reason for the difference of location having been explained, the structure can be described.

Referring first to the upper working space, 3| is the air inlet passage leading to seat 32 of inlet valve 33, from which passage 34 leads to the upper working space above piston l2. Removable cap 35 is modified according to the invention. It is counterbored from the top and threaded at 36 to receive relief valve housing 37, having a restricted outlet port 38 (see Fig. 3). Through ports 40 are formed in the cap. Screwed into the lower end of housing 31 is member 39 having a center port 4| substantially larger than port 33 and a surrounding seat 42 for a disc valve 33. A coil compression spring 44 acts through pressure centering seat 45 to urge valve 43 closed.

Two intermediate valves 46 coact with seats 4'! to permit flow from the upper working space of cylinder l3 to passage 48 and hence to the upper working space of cylinder l6. These valves close against reverse flow. They follow standard construction.

Referring now to the lower working space, the inlet air passage 5| leads through ports in cage 52 (threaded into head 29) to the seat 53 of inlet valve 54. This valve controls inlet to the lower working space in cylinder 3, and follows standard construction. One of the two intermediate valves is unchanged, but the cage of the other is modified to carry a relief valve. In Fig. 1 the valve 55 coacts with a seat in a ported cage 55 which is screwed into head 29 as usual. The other valve 57 coacts with a seat 58 in a modified cage 59, shown in detail in Fig. 2. Valves .55 and 5'! permit parallel flows from space 8|, in communication with the lower working space of cylinder I3, to passage 62 which leads to the lower working space in cylinder l6.

Referring now to Fig. 2, the modified cage 59 is counterbored and threaded at its outer end to receive a relief valve housing 31a, identical with Through ports 60 are formed in cage 59 to lead to the relief valve. The parts 31a to 39a and Ma to 45a correspond to similarly numbered parts in Fig. 3, in fact, these parts comprise identical and interchangeable relief valves. Each permits flow from a corresponding low pressure working space to atmosphere.

In each relief valve the spring 44 (44a) offers such resistance that valve 43 (43a) will not open until the normal intermediate pressure is definitely exceeded, but will open at a pressure below that which would stall the high pressure steam piston under the minimum boiler pressure encountered in service. Port 38 (38a) restricts the outflow to a rate so low that even if the corresponding valve 43 (4311) should fail to seat the pump will still operate with substantial capacity. By making port 38 smaller than port 4| the tendency of the relief valves to squeal is virtually eliminated.

The discharge connection of the compressor is shown at 63. The discharge valves are shown at 64 and 65 and are of familiar form.

The structure, above described in considerable detail, is intended to be illustrative and not limiting.

The invention has given complete success in service, is easily applied to new or old compressors at small expense, does not impair the emciency of the compressor, and does not introduce a component Whose failure would disable the compressor.

What is claimed is:

1. The combination of a double acting stage gas compressor including high and low pressure steam motor pistons respectively connected to first and second stage compressor pistons, and compound steam distributing valve gear arranged to be reversely shifted by the high pressure steam piston near the limits of its working strokes; inlet and intermediate valve units for the working spaces of the compressor first stage, said valve units for one working space including removable caps under which the valves are mounted, and units for the other working space including removable cages in which the valves are mounted; and relief valves for relieving excess pressure in said first stage working spaces, one mounted in an inlet valve cap and the other in an intermediate valve cage, said relief valves being arranged to open at a pressure higher than the maximum reached in said spaces during normal operation and lower than that which will stall the high pressure steam piston under the lowest available high pressure steam pressure.

2. Relief valve means for cross compound steam driven stage gas compressors of the type in which an inlet valve for one first stage working space is mounted under a removable cap giving access to the working space, and an intermediate discharge valve for the other first stage working space is mounted in a removable cage giving communication with said other working space; said means including a substitute inlet valve cap and a substitute intermediate valve cage, each provided with a loaded relief valve arranged to vent excessive pressures from the corresponding working space.

3. Relief valve means for cross compound steam driven stage gas compressors, of the type in which an inlet valve for one first stage working space is mounted under a removable cap giving access to the working space, and an intermediate discharge valve for the other first stage working space is mounted in a removable cage giving communication with said other working space; said means including a substitute inlet valve cap and a substitute intermediate valve cage, each provided with a loaded relief valve arranged to vent excessive pressures from the corresponding working space; and throttling means associated with said relief valves to limit the flow therethrough.

4. A relief valve structure for the first stage of a stage compressor having automatic valve mechanisms including an element removable to provide for removal of such automatic valve, said element afiording communication with a working space, said relief valve structure comprising a substitute element for said removable element and a loaded relief valve carried thereby and arranged to communicate therethrough with such working space.

5. The combination defined in claim 4 in which said removable element is a cap overlying an inlet valve.

6. The combination defined in claim 4 in which said removable element is a cage carrying a seat for a low pressure discharge valve.

. CHARLES A. CAMPBELL. 

